1. Field of the Invention
The present invention relates to a toroidal-type continuously variable transmission which can be used in an automobile and in various kinds of industrial equipment.
2. Related Background Art
As a transmission for an automobile, a toroidal-type continuously variable transmission schematically shown in FIGS. 5 and 6 is being studied. As disclosed, for example, in Japanese Utility Model Appln. Laid-Open No. 62-71465, in a typical toroidal-type continuously variable transmission, an input disc 2 is supported coaxially with an input shaft 1 and an output disc 4 is fixed to an end of an output shaft 3 which is arranged coaxially with the input shaft 1. In a housing which contains the toroidal-type continuously variable transmission, trunnions 6 which are rocked around respective pivots 5 are provided. Each pivot 5 is arranged in a skewed position with respect to both the input shaft 1 and the output shaft 3. The pivot 5 of each trunnion 6 has ends that project from the side surfaces of the trunnion 6. Each trunnion 6 has a mobile shaft 7 where base portion is supported at its center. By rocking the trunnions 6 around their respective pivots 5, tilt angles of the mobile shafts 7 can be varied. Each mobile shaft 7 supported by a trunnion 6 rotatably supports a power roller 8, which is held between the input disc 2 and the output disc 4. The inner surface 2a of the input disc 2 and the inner surface 4a of the output disc 4, which face with each other, are concave surfaces of revolution each having a cross-sectional outline containing arcs whose centers coincide with the axes of the pivots 5. Thus spherically shaped peripheral surface 8a of each power roller 8 can fittingly come into contact with the inner surfaces 2a and 4a of the discs 2 and 4.
A loading-cam-type pressure device 9 is provided between the input shaft 1 and the input disc 2 in order to elastically press the input disc 2 toward the output disc 4. This pressure device 9 comprises a cam plate 10 rotated together with the input shaft 1, a holder 11 and plurality of (for example, four) rollers 12. A cam surface 13 which is radially corrugated is formed on one surface (left surface, in FIGS. 5 and 6) of the cam plate 10. A similar cam surface 14 is formed on the outer surface (right surface, in the FIGS. 5 and 6) of the input disc 2. Both cam surfaces 13 and 14 and the holder 11 support said plurality of rollers 12 so that the rollers can roll around axes perpendicular to the axis of the input shaft 1 and rotate around the axis of the input shaft 1.
At the time of operation of the toroidal-type continuously variable transmission having the above-mentioned construction, when the cam plate 10 together with the input shaft 1 is rotated, the cam surface 13 presses the plurality of rollers 12 against the cam surface 14 formed on the outer surface of the input disc 2. As a result, the input disc 2 is pressed against the power rollers 8. At the same time, as the cam surfaces 13 and 14 are engaged with each other via the plurality of rollers 12 in between, the input disc 2 is given torque from the input shaft 1 and is rotated. The torque of the input disc 2, then, is transmitted through the power rollers 8 to the output disc 4. Thus the output shaft fixed to the output disc 4 is rotated.
In order to change the ratio of the rotating speeds of the input shaft 1 and the output shaft 3, trunnions 6 are moved around their respective pivots 5. More specifically, in order to decelerate the automobile, the mobile shafts 7 are tilted as shown in FIG. 5, in which the peripheral surfaces 8a of the power rollers 8 come into contact with the inner surface 2a of the input disc 2 at positions near the center of the input disc 2 as well as with the inner surface 4a of the output disc 4 at positions near the periphery of the output disc 4.
On the other hand, in order to accelerate the automobile, the trunnions 6 are moved around their respective pivots 5 to tilt the mobile shafts 7 as shown in FIG. 6, in which the peripheral surfaces 8a of the power rollers 8 come into contact with the inner surface 2a of the input disc 2 at positions near the periphery of the input disc 2 as well as with the inner surface 4a of the output disc 4 at positions near the center of the output disc 4. When the tilt angles of the mobile shafts 7 are arranged somewhere between the states shown in FIGS. 5 and 6, various ratios of the rotating speed of the input shaft 1 and that of the output shaft 3 can be obtained.
FIGS. 7 to 10 show a more concrete construction of a toroidal-type continuously variable transmission disclosed in microfilms of Japanese Utility Model Appln. No. 63-69293 (Japanese Utility Model Appln. Laid-open No. 1-173552). As shown in FIG. 7, an input disc 2 and an output disc 4 are rotatably supported through respective needle bearings 16 around a tubular input shaft 15. A cam plate 10 having splines is engaged with the splines formed around the outer peripheral surface near an end portion (left end, in FIG. 7) of the input shaft 15, and the cam plate 10 is prevented from shifting leftward and retreating from the input disc 2 by a collar portion 17. A loading-cam-type pressure device 9 for pressing the input disc 2 toward the output disc 4 and transmitting the torque of the input shaft 15 to the input disc 2 to rotate the input disc 2 consists of said cam plate 10 and rollers 12. The output disc 4 is engaged with an output gear 18 through keys 19 so as to be rotated together with the output gear 18.
A pair of trunnions 6 is supported by a pair of support plates 20 so that the trunnions can be rocked, and shifted in a direction (which is vertical to the paper of FIG. 7, and is a right-left direction in FIG. 8). A mobile shaft 7 is supported in a round hole 23 formed in the middle of each trunnion 6. Each mobile shaft 7 consists of a support shaft portion 21 and a pivot portion 22, wherein the axes of these portions are parallel to each other and eccentric from each other. Each support shaft portion 21 is rotatably supported by a needle bearing 24 in its corresponding round hole 23. Power rollers 8 are rotatably supported around their respective pivot portions 22 via respective needle bearings 25.
The end faces of the two mobile shafts 7 face to each other and are rotation-symmetrically arranged with respect to the axis of the input shaft 15. The pivot portions 22 of the mobile shafts 7 are eccentric from their respective support shaft portions 21 in the direction toward which the power rollers 8 are pushed by the rotating input and output discs 2 and 4 (horizontally opposite directions in FIG. 8). Each pivot portion 22 deviates from the corresponding support shaft portion 21 along a line whose direction is substantially vertical to that of the axis of the input shaft 15. In this arrangement, the power rollers 8 are supported while allowed to shift a little in the direction of the axis of the input shaft 15. As a result, even if the power rollers 8 are assembled in the apparatus with deviations in the axial directions of the input shaft 15 because of, for example, dimensional inexactitude of the components, excessive stress caused by such deviations can be absorbed without loading it to the components.
Between the outer surface of each power roller 8 and the inner surface of each trunnion 6, a thrust ball bearing 26 and a thrust needle bearing 27 are provided in this sequence from the side of the outer surface of the power roller 8. The thrust ball bearings 26 absorb the load given on the power rollers 8 in the thrust direction, while rotatably supporting the power rollers 8. Each thrust ball bearing 26 consists of plurality of balls 29, an annular holder 28 for holding the balls 29 so that they can roll in the holder 28, and annular outer ring 30. The inner raceway of each thrust ball bearing 26 is formed on the outer surface of the corresponding power roller 8, while the outer raceway of each thrust ball bearing 26 is formed on the inner surface of the corresponding outer ring 30.
Each thrust needle bearing 27 consists of a race 31, a holder 32 and needles 33, as shown in FIGS. 9 and 10. The race 31 and the holder 32 are assembled so as to shift a little from each other around the circular opening. The race 31 has an annular portion 34a and a protruding portion 35a which radially protrudes from part of the periphery of the annular portion 34a. Similarly, the holder 32 has an annular portion 35b and a protruding portion 34b.
Thus constructed thrust needle bearings 27 are held between the inner surfaces of the trunnions 6 and the outer surfaces of the outer rings 30 with the races 31 in contact with the inner surfaces of the corresponding trunnions 6. The protruding portions 35a and 35b are arranged to be on the side of the pivot portions 22 eccentric from their respective support shaft portions 21. Thus arranged thrust needle bearings 27 absorb thrust loads given from the power rollers 8 to the outer rings 30, while allowing the pivot portions 22 and the outer rings 30 to be rocked around their respective support shaft portions 21.
Further one end portion (lower end portion, in FIG. 8) of each trunnion 6 is connected with a drive rod 36. Drive pistons 37 are fixed to the outer peripheral surfaces of the intermediate parts of respective drive rods 36. These drive pistons 37 are provided inside respective oiltight drive cylinders 38.
In the toroidal-type continuously variable transmission having the above-mentioned construction, the torque of the input shaft 15 is transmitted via the pressure device 9 to the input disc 2. Then, the torque of the input disc 2 is transmitted via the pair of power rollers 8 to the output disc 4. Finally, the torque of the output disc 4 is output via the output gear 18.
In order to change the ratio of the rotating speeds of the input shaft 15 and the output gear 18, said pair of drive pistons 37 are moved in the opposite directions. By moving the driving pistons 37 in this way, said pair of trunnions 6 are rocked in opposite directions around their respective pivots 5 supported by the support plates 20. Accordingly, the directions of the tangential force acting at each contact position of the power rollers 8 and the inner surfaces 2a and 4a of the input and output discs 2 and 4 are changed.
As a result, as shown in FIGS. 5 and 6, the contact positions of the peripheral surfaces 8a of the power rollers 8 and the inner surfaces 2a and 4a of the discs 2 and 4 change to vary the ratio of rotating speeds of the input shaft 15 and the output gear 18.
When the tilt angles of respective mobile shafts 7 are changed in order to vary the ratio of rotating speeds of the input shaft 15 and the output gear 18, the mobile shafts 7 are rocked a little around their support shaft portions 21. At the same time, the outer surface of the outer ring 30 of each thrust ball bearing 26 changes its position with respect to the inner surface of the corresponding trunnion 6. Because of the needle bearing 27 provided between the outer surface of the outer ring 30 and the inner surface of the trunnion 6, only small force is needed to shift the outer ring 30 along on the inner surface of the trunnion 6. In other words, only small force is needed to change the tilt angles of the mobile shafts 7.
As for the toroidal-type continuously variable transmission having the above-mentioned construction and functions, however, the present inventors have found that the durability of the outer ring 30 of the thrust ball bearing 26 is not sufficient and that this problem must be solved in order to practically apply the toroidal-type continuously variable transmission to an automobile. More particularly, the thrust needle bearing 27, which has the race 31 and the holder 32 as shown in FIG. 9 and which is held between the outer surface of the outer ring 30 of the thrust ball bearing 26 and the inner surface of the trunnion 6, cannot effectively absorb the thrust load given to the outer ring 30 through the thrust ball bearing 26.
In other words, the thrust needle bearing 27 used in the conventional toroidal-type continuously variable transmission is effective only for the purpose of smoothing the relative shift between the outer ring 30 and the trunnion 6, and not for the purpose of reinforcing the outer ring 30 against said thrust load. Also, part of the outer ring 30 is not covered with the thrust needle bearing 27 even when the protruding portions 34a and 34b are formed in the race 31 and the holder 32 as shown in FIG. 9. The thrust load given to said uncovered part of the outer ring 30 cannot be absorbed by the needles 33 held in the holder S2.
On the other hand, in the thrust ball bearing 26, the entire periphery of the outer ring 30 receives thrust load from the plurality of balls 29. So the outer ring 30 gets bending stress coming from the thrust load along the border line between the part which is covered with the thrust needle bearing 27 and the uncovered part. When the toroidal-type continuously variable transmission is used in an automobile, the outer rings 30 are repeatedly subjected to very strong bending stress as the plurality of balls 29 rotates around the pivot portions 22. As a result, in a relatively short time the outer rings 30 are damaged. For example, they often get cracked or flaked. Thus, sufficient durability of the toroidal-type continuously variable transmission cannot be obtained.
The present inventors carried out experiments on the construction employing thrust needle bearings similar to those in the prior art, and found that the outer raceways formed on the inner surfaces of the outer rings 30 could soon be flaked to cause rattle in the thrust ball bearings 26. FIG. 11 shows a thrust needle bearing examined by the inventors, which comprises an arcuate main holder 39, annular auxiliary holders 40a and 40b, and plurality of needles 33 rotatably held in the holders 39, 40a and 40b. The main holder 39 is set around the support shaft portion 21 of the mobile shaft 7, while the auxiliary holders 40a and 40b are arranged corresponding to the pivot portion 22 which is eccentric from the support shaft portion 21.
Two arcuate regions and one annular region which have the same width as the needles 33 and include rows of needles 33 form the genuine load-absorbing region where the thrust load can be absorbed by the needles 33. In the construction shown in FIG. 11, the gap between said two arcuate regions as well as the gap between the arcuate region corresponding to the auxiliary holder 40b and the annular region corresponding to the main holder 39 can be included in said load-absorbing region. About 65% of the entire circumference of a pitch circle a of the thrust ball bearing 26 falls in the load-absorbing region of the thrust needle bearing examined in the experiment when seen from the direction of the axis of the pivot portion 22. In FIG. 11, the part of the circumference falling in the load-absorbing region is indicated by a thicker portion of the broken line, while the other part is indicated by a thinner portion of the broken line.
As a result, it is confirmed that more than 65% of the entire circumference of the pitch circle a of the balls 29 of the thrust ball bearing 26 has to fall in the load-absorbing region to obtain sufficient durability of the outer ring 30.